The coordinates are Latitude and Longitude marks on the map. The -158 is on the left side and 22S is in the lower half. So together they will be in the lower left quarter of the map. You have to hold your mouse really steady and move it very slowly to get it to stop on the right spot then you click on the spot.
Perhaps you can move along the Equator to the left of center until you find -158, then move your cursor down while holding the -158 setting until you hit 22S.
Edited on 11/21/2008 at 2:35:09 AM PST